1. Field of the Invention
The present invention relates to an image forming apparatus configured to form images using toners having substantially the same hue but different densities and a method for controlling the image forming apparatus.
2. Description of the Related Art
An image forming apparatus configured to form images using electrography includes a charging unit capable of uniformly charging a photosensitive surface of a photosensitive drum. The image forming apparatus also includes a latent-image forming apparatus configured to form latent images corresponding to image information on the charged photosensitive surface of the photosensitive drum and a developing unit configured to develop the latent images with developers. In addition, the image forming apparatus includes a transferring unit configured to transfer the developed latent images onto a recording material and a fixing unit configured to fix the transferred latent image on the recording material.
In general, for the developers (toners), one type of toner having a predetermined density is used for each color, i.e., cyan, magenta, yellow, or black. However, when toners having the same density are used, the amount of toner used in the highlighted areas of an image (i.e., low density areas) is reduced. For this reason, there are difficulties in the reproducibility of the gradation (density gradation) of the image data. Recently, the need for high quality image formation has grown. To meet this need, an image forming apparatus that uses a greater number of toner colors compared with previously known image forming apparatuses capable of forming four-color images has been proposed. More specifically, an electrographic image forming apparatus using toners having substantially the same hue but different densities is described in Japanese Patent Laid-Open Nos. 2001-290319 and 2004-145137.
Many of such image forming apparatuses use six different toner colors, i.e., the four colors of cyan, magenta, yellow, and black and two additional colors of light cyan and light magenta. The colorants included in light cyan and light magenta toners have the same spectral characteristics as those of regular cyan and magenta toners, respectively, but the amount of colorant included in the lighter toners is smaller. Hereinafter, regular cyan and magenta toners are referred to as ‘dark toners,’ and light cyan and light magenta toners are referred to as ‘light toners.’ Moreover, an image signal controlling the output of a dark toner is referred to as a ‘dark toner image signal,’ and an image signal controlling the output of a light toner is referred to as a ‘light toner image signal.’
FIG. 19 is a graph showing the relationships among the color density indicated by an input signal corresponding to an input image, the amounts of dark and light toners applied to a sheet of recording paper, and the output densities of dark and light toners. A solid line T1 and a dotted line T2, shown in FIG. 19, represent the amount of light toner and dark toner, respectively, applied on a sheet of recording paper to reproduce the color density indicated by an input signal corresponding to an input image. A straight line m represents the optimal output density corresponding to the color density indicated by an input signal corresponding to an input image. The amounts of dark toner and light toner applied to a sheet of recording paper to reproduce the color density indicated by an input signal corresponding to an input image are determined so that the graph representing the relationship between the density of the input image and the density of an output image formed with dark and light toners has an optimal line shape. When the maximum density of an image corresponding to an input image signal is set as 1.8, areas ranging from highlighted areas (low density areas) to intermediate areas are formed only with light toner so as to reduce the granulated effect of the image. Areas ranging from intermediate areas to high density areas having a density of 0.9 or more are formed with both dark toner and light toner wherein as the density increases the amount of light toner used is reduced and the amount of dark toner used is increased so as to reduce the total amount of toners applied on the surface of the recording paper.
However, when the output characteristics of dark and light toners are changed for the image forming apparatus configured to form images using dark and light toners, the problems identified below may occur.
When resistance of the surface layer of the photosensitive drum and the triboelectricity of the toners decrease because of the environment and/or conditions of the image forming apparatus, the contrast voltage Vcont decreases. As a result, the amount of toners attached to the surface of a sheet of the recording paper changes, causing the output density to be reduced.
More specifically, a curved line 1 in FIG. 19 represents a reduction in the amount of dark toner applied to a sheet of recording paper. The output density at this time is represented by a curved line n. As is apparent from the curved line n, the output density suddenly changes in the area having an intermediate density (i.e., in the area where the density of the image is around 0.9) where dark toner starts to be added for image formation. Therefore, images having areas with intermediate densities may exhibit an unsmooth change in gradation and/or include false outlines.